High-voltage metal-oxide-semiconductor (HVMOS) devices are widely used in many electrical devices, such as CPU power supplies, power management systems, AC/DC converters, etc.
FIG. 1 illustrates conventional HVMOS 2, which includes gate oxide 10, gate electrode 12 on gate oxide 10, drain region 4 in a high-voltage n-well (HVNW) region (also referred to as drift region), and source region 6 in a high-voltage p-well (HVPW) region. Field oxide 8 spaces drain region 4 and gate electrode 12 apart so that a high drain-to-gate voltage can be applied.
Breakdown voltage and on-resistance are two of the key parameters of HVMOS devices, and increasing breakdown voltage and lowering on-resistance are among the major goals in the design of HVMOS devices. Typically, the breakdown voltage of an HVMOS device may be improved by increasing the distance between drain region 4 and gate electrode 12, hence increasing the size of the HVMOS device. Alternatively, the breakdown voltage may be increased by reducing the impurity concentration in the drift region HVNW. However, larger sizes of HVMOS devices result in greater power consumption, while reduced impurity concentrations result in increased on-resistances.
FIG. 2 illustrates another conventional structure for reducing the on-resistance. The structure is similar to the structure shown in FIG. 1, except a pre-HVNW region is formed underlying the HVNW region. Typically, in the formation of the HVMOS shown in FIG. 2, a substrate is provided, and the pre-HVNW region is doped on a surface of a substrate. An epitaxial growth is then performed to form an epitaxial layer, and the HVNW region and HVPW region are formed by implanting the epitaxial layer with desired dopants. The resistance of the pre-HVNW region parallels the resistance of the HVNW regions, and thus the on-resistance of the HVMOS device is reduced.
The HVMOS device shown in FIG. 2, however, has drawbacks. Pre-HVNW region 16 serves as the collector of the parasitic bipolar junction transistor (BJT) 14. The base resistor 18 of BJT 14 is located in the original substrate and the epitaxial region, both having low impurity concentrations. Therefore, the resistance of the parasitic base resistor 18 is high. This causes the voltage applied on the base of BJT 14 to be undesirably high, and hence BJT 14 will be easily turned on. As a result, the breakdown voltage of the MOS device is degraded.
What is needed in the art, therefore, is a HVMOS device having a high breakdown voltage and a low on-resistance, and corresponding methods for forming the same.